1. Field of the Invention
The present invention relates to a hydrogen generator and a method of shutting down its operation as well as a fuel cell system, and more particularly to a hydrogen generator in which an interior of a reformer can be purged so that a combustible gas emitted to its outside can be appropriately processed and a method of shutting down its operation as well as a fuel cell system.
2. Description of the Related Art
There are known reformers (reformer systems) that generate a reformed gas containing hydrogen as its main component by a steam reforming reaction with steam and an organic material such as natural gas or naphtha.
Among reforming catalyst bodies that contribute to this steam reforming reaction is one in which a carrier such as alumina carries a Ni-based or a Ru-based catalyst. The performance of the reforming catalyst body, however, greatly depends on the atmosphere of the surrounding environment; for example, the Ni-based catalyst can exhibit its expected performance in a reduced state but its catalytic activity deteriorates in an oxidized state. Likewise, the Ru-based catalyst tends to volatilize easily and therefore shows degraded catalytic activity in an oxidized state at high temperature.
Particularly during a shutdown period of the reformer, there is a possibility of air (oxygen) intruding therein from the outside of the equipment due to the depressurizing effect originating from the temperature decrease of a high-temperature remaining gas in the reformer, and there is an increasing concern over the deterioration of the reforming catalyst body by oxidation. Therefore, it is necessary to take a measure to inhibit oxidation of catalysts during a shutdown period of the reformer. For this purpose, the following method has been adopted; after ceasing maintaining the temperature of the reforming catalyst body at a high reforming temperature (at 700° C., for example), purging is performed in such a manner that an inert gas such as nitrogen is introduced into the interior of the reformer and the interior of the reformer is filled with nitrogen in order to replace the high-temperature remaining gas in the interior of the reformer; thereafter, the upstream and downstream of the reformer are closed by sealing valves to inhibit deterioration of the reforming catalyst body.
Another method in which a hydrogen generator is purged with a feed gas for the hydrogen generator itself in place of nitrogen has been proposed as remedies for the increased cost and complexity associated with obtaining nitrogen. (See, for example, Japanese Unexamined Patent Publication 2000-95504.) With this method, the interior of the reformer can be appropriately purged without using nitrogen after shutting down the reformer, and cost reduction and simplification of the gas supply system are made possible.
Although the foregoing publication does not mention post-processing of a combustible purge gas that is forced out by purging the reformer, a post-processing method normally assumed for effectively utilizing existing equipment is such that a purge gas is returned to a burner for heating the reforming catalyst body of the reformer and then combustion-treated with the burner.
When the interior of the reformer is purged with a feed gas, it is important to reduce the temperature of the reforming catalyst body as quickly as possible so as not to thermally decompose the feed gas and to complete the purging operation for the reformer within a minimum time, from the viewpoint of saving the energy required for the purging. Nevertheless, the above-described combustion of the purge gas in the burner functions to reduce the rate of the temperature decrease of the reforming catalyst body and thereby delays the purging of the reformer, leading to an increase in energy loss in the hydrogen generator. Moreover, if a purge gas that is useful as a power generation gas is burnt in the burner when purging the reformer, the amount of heat that is to be produced by this combustion cannot be utilized effectively, and effective utilization of energy cannot be attained.